The present invention relates to a method of forming a boss of a boss-integrated sheet metal member. The boss-integrated sheet metal member according to the present invention is outwardly fittingly fixed to a rotary shaft, a stationary shaft, or the like. Among the boss-integrated sheet metal members of the present invention, a member in which a cylindrical portion is formed at an outer peripheral edge may be used as a pulley for a flat belt, and that in which a V groove or poly-V grooves are formed on a cylindrical portion at an outer peripheral edge may be used as a pulley for a V belt or poly-V belts.
As a method of forming a boss of a boss-integrated sheet metal member, conventionally, known are a cold forging method, a drawing method, a burring method, and the like.
Among the conventional methods, the cold forging method forms a boss by using a plastic flow of the material itself. In the method, when a boss in which the projection degree (projection height) is larger as compared with the inner diameter, such as when a crank shaft pulley is to be formed by using a material of a large thickness, it is difficult to form a boss having a predetermined projection height, even though a large-size press machine of the 2,000 to 2,500 tons class is used.
In the drawing method, a material flow due to the drawing occurs, and hence the thickness of the boss tends to be insufficient, with the result that a satisfactory strength may not be obtained.
In the burring method, an edge portion of a circular hole which was previously formed by punching using a punch or the like is raised. When a boss having a small inner diameter is formed, therefore, there arises a case in which the height of the boss is not sufficiently ensured. Accordingly, the range in which a boss can be formed is largely limited by the relationship between the inner diameter and the height.
It is an object of the present invention to provide a method of forming a boss of a boss-integrated sheet metal member which can form a boss having a desired inner diameter, a desired thickness, a desired height, and a satisfactory strength, and to a boss of a boss-integrated sheet metal member by using a small press machine, and which can accurately form a boss at a center position.
The method of forming a boss of a boss-integrated sheet metal member of the present invention is a method comprising: a boring step of opening a circular hole at a center portion of a circular sheet metal blank; a curving step of shaping the circular blank into a shape which is projected toward a projection side of a boss; and a bending step of restricting a radially outward spread of an outer peripheral edge portion of the circular blank which has undergone the curving step, and pressing an inclined portion between the outer peripheral edge portion of the circular blank and the circular hole in a direction opposite to the projection direction, thereby bending the inclined portion into a recessed shape to form a case-like boss in which the circular hole serves as a tip end opening, wherein the boring step and the curving step are sequentially conducted in an arbitrary order, and the bending step is then conducted while the circular hole is positioned to a reference position.
In the method, whereby boring and the curving step are sequentially conducted in an arbitrary order, includes both cases where the boring step is conducted and the curving step is then conducted, and where the curving step is conducted and the boring step is then conducted.
The method of forming a boss of a boss-integrated sheet metal member of a variant of the present invention is a method comprising: a boring step of opening a circular hole at a center portion of a circular sheet metal blank; a curving step of shaping the circular blank into a shape which is projected toward a projection side of a boss; and a bending step of restricting a radially outward spread of an outer peripheral edge portion of the circular blank which has undergone the curving step, and pressing an inclined portion between the outer peripheral edge portion of the circular blank and the circular hole in a direction opposite to the projection direction, thereby bending the inclined portion into a recessed shape to form a case-like boss in which the circular hole serves as a tip end opening, wherein the boring step and the curving step are conducted in the same molds.
In the above-mentioned variants of the invention, each of the curving step and the bending step corresponds to a step of conducting a kind of a bending process. Therefore, the reduction of the thickness of the circular sheet metal blank due to a plastic flow of material in the curving step or the bending step, and the reduction in strength due to the thickness reduction are suppressed. Even when the circular blank has a large thickness, a boss can be formed by using a relatively small press machine. In the bending step, moreover, a radially outward spread of an outer peripheral edge portion of the circular blank which has undergone the curving step is restricted, and hence the material is prevented from flowing radially outwardly. This also serves to prevent the thickness of the boss from being reduced. In the bending step, an inclined portion between the outer peripheral edge portion of the circular blank which has undergone the curving step and the circular hole is pressed in the direction opposite to the projection direction, thereby bending the inclined portion into a recessed shape. Therefore, the height of the boss can be freely determined depending on the position of the inclined portion where the pressurization is applied. Specifically, when a part which is close to the center of the inclined portion is pressed, a boss of a small height is formed, and, when a part which is remote from the center of the inclined portion is pressed, a boss of a large height is formed.
According to the first variant of the present invention, particularly, the boring step and the curving step are sequentially conducted in an arbitrary order, and the bending step is then conducted while the circular hole opened in the boring step is positioned to a reference position. Therefore, the boss is prevented from deviating from the center in the bending step, and the boss can be accurately formed at a center position.
According to the other variant of the present invention, the boring step and the curving step are conducted in the same molds. Therefore, the circular hole can be accurately formed at a center position of the curved circular blank. In the formation of a boss, this improvement of the accuracy of the center position of the circular hole enables the boss to be accurately formed at. the center position.
In the method of forming a boss of a boss-integrated sheet metal member of a further variant of the present invention, in each of the above-mentioned methods, the bending step is conducted while the circular hole of the circular blank which has undergone the boring step and the curving step is fitted onto a projection formed on a mold for conducting the bending step, thereby positioning the circular hole to a reference position.
According to the method, when the circular hole of the circular blank is to be positioned to the reference position, it is required only to fit the circular hole onto the projection of the mold which is used for conducting the bending step. Therefore, the operation of positioning the circular hole can be conducted easily and surely, the boss is surely prevented from being deviated from the center in the bending step, and the boss can be accurately formed at the center position.
In the method of forming a boss of a boss-integrated sheet metal member of a still further variant of the present invention, in each of the above-mentioned methods, the bending step is conducted by plural stages.
According to the method, the inclined portion of the circular blank which has undergone the curving step can be bent without producing undue stress.
The method of forming a boss of a boss-integrated sheet metal member of a still further variant of the present invention relates to a method in which the boring step and the curving step are sequentially conducted in an arbitrary order, and in which, after the boring step is conducted, the curving step is conducted while the circular hole opened in the circular blank in the boring step is fitted onto a projection formed on a mold for conducting the curving step, thereby positioning the circular hole to a reference position.
According to the method also, when the circular hole of the circular blank is to be positioned to the reference position, it is required only to fit the circular hole onto the projection of the mold which is used for conducting the curving step. Therefore, the operation of positioning the circular hole can be conducted easily and surely. The curving step is conducted while positioning the circular hole opened in the boring step, to the reference position. Consequently, the center position of the boss is surely prevented from being deviated in the curving step.